IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 29, NO. 24, DECEMBER 15, 2017

2167

Pressure Dependence of Fiber Bragg Grating
Inscribed in Perfluorinated Polymer Fiber
Ryo Ishikawa, Heeyoung Lee , Amédée Lacraz, Antreas Theodosiou , Student Member, IEEE,
Kyriacos Kalli, Member, IEEE, Yosuke Mizuno , Senior Member, IEEE,
and Kentaro Nakamura, Member, IEEE
Abstract— We investigate the hydrostatic pressure dependenceof the Bragg wavelength of a fiber Bragg grating (FBG)
inscribed in a perfluorinated graded-index (PFGI-) polymer
optical fiber (POF) at 1550 nm. At 0.5 MPa, the Bragg wavelength
increased with time and became almost constant ∼150 min later.
Such a long time constant probably originates from the uniquestructure of the PFGI-POF, which has a thick overcladding
around its core and cladding. The pressure-dependence coefficient
without considering the time constant was estimated to be
1.3 nm/MPa; this is over five times the values of other typesof POF-FBGs. This indicates that by removing the overcladdingof the PFGI-POF, fast-response high-sensitivity pressure sensing
will be feasible. Once the Bragg wavelength became constant
at 0.5 MPa, the pressure-dependence coefficient of the Bragg
wavelength was measured to be −0.13 nm/MPa, the absolutevalue of which was comparable with those of other POF-FBGs,
but with an opposite sign.
Index Terms— Polymer optical fibers, fiber Bragg gratings,
pressure sensing, temporal variations.

I. I NTRODUCTIONPTICAL fiber sensing techniques have been vigorously
studied to perform various physical, chemical, andbiological measurements [1], [2]. The sensing configurations
are classified into two types: distributed sensing and single(or multiple-) point sensing. The former includes strain and/or
temperature sensing based on Brillouin scattering [3]–[5];although the distributed measurement capability is a great
advantage, its sensitivity is not sufficiently high for some
applications (such as small-strain detection of 10 με). The
latter includes sensing based on fiber Bragg gratings (FBGs)[6]–[15], the sensitivity of which is generally higher than thatof the former at the cost of the limited number of sensing
points. Here, we focus on FBG sensors.

O

Manuscript received October 10, 2017; revised October 20, 2017; accepted
October 24, 2017. Date of publication October 27, 2017; date of current
version November 10, 2017. This work was supported in part by JSPS
KAKENHI under Grant 17H04930 and Grant 17J07226, in part by the Japan
Gas Association, in part by the ESPEC Foundation for Global Environment
Research and Technology, and in part by the Association for Disaster
Prevention Research. (Corresponding author: Yosuke Mizuno.)
R. Ishikawa, H. Lee, Y. Mizuno, and K. Nakamura are with the Instituteof Innovative Research, Tokyo Institute of Technology, Yokohama 226-8503,
Japan (e-mail: rishikawa@sonic.pi.titech.ac.jp; hylee@sonic.pi.titech.ac.jp;
ymizuno@sonic.pi.titech.ac.jp; knakamur@sonic.pi.titech.ac.jp).
A. Lacraz, A. Theodosiou, and K. Kalli are with the Nanophotonics
Research Laboratory, Cyprus University of Technology, Limassol 3036,
Cyprus (e-mail: amedee.lacraz@cut.ac.cy; theodosiou.antreas@gmail.com;
kyriacos.kalli@cut.ac.cy).
Color versions of one or more of the figures in this letter are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2017.2767082

Fig. 1. (a) Experimental setup. ASE: amplified spontaneous emission; OSA:
optical spectrum analyzer. (b) Photograph of hydraulic system.

The interest in using FBGs as point sensors is limited
not only to strain and temperature [8] but to humidity [9],refractive index [10], and many other physical parameters.
In particular, pressure sensing is one of the major targets, anda number of relevant results have been reported using FBGs
inscribed in glass optical fibers [11]–[15]. As the intrinsic
pressure sensitivity of the FBG in glass fibers is relatively
small [15], higher-sensitivity pressure sensing has been basically achieved by converting pressure into strain [11]–[14].
In the meantime, as polymeric materials are generally softer
than glass materials, intrinsic pressure sensitivities have beenenhanced using FBGs inscribed in some types of polymer
optical fibers (POFs) [16]–[18]. For instance, at telecom
wavelengths, FBGs in a single-mode (SM-) poly(methyl
methacrylate) (PMMA-) POF [16] and a multimodemicrostructured (MM-m) POF [17] are reported to havepressure sensitivities of ∼0.25 nm/MPa (corresponding toa fractional sensitivity (Bragg wavelength shift divided
by the initial Bragg wavelength for fair comparison) of∼163 × 10−6 /MPa) and ∼0.1 nm/MPa (corresponding to∼64 × 10−6 /MPa), respectively, the absolute values of which
are approximately ∼82 and ∼32 times larger than the typical value of an FBG in a silica single-mode fiber (SMF)(−3.1 pm/MPa, corresponding to −2.0 × 10−6 /MPa) [15].
However, such conventional POF-FBGs suffer from extremely
high propagation loss at 1550 nm.
There is interest in the use of perfluorinated graded-index(PFGI-) POFs, which have been developed for short distancecommunication systems (relatively low propagation loss evenat 1550 nm) [19]. The PFGI-POFs are only available asmultimode fibers and are not photosensitive in the UV wavelength region; both these issues have complicated the FBG
inscription. Recently, some research groups have developed
inscription techniques for FBGs in PFGI-POFs [20]–[23].

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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 29, NO. 24, DECEMBER 15, 2017

Fig. 2. (a, b) Measured optical spectra of the ASE output and the reflected light, respectively. (c) Magnified view of the red-circled part in (b), around the
FBG-induced peaks.

At 1550 nm, amplified spontaneous emission (ASE) can bedirectly used as a wideband light source for interrogating
the Bragg wavelength. Moreover, the core refractive index of
PFGI-POF-FBGs is close to that of water, which is beneficial
for bio-sensing applications [24]. To date, the strain, temperature, and humidity sensing characteristics of PFGI-POF-FBGs
have been investigated [20], [25], [26], but no reports havebeen provided regarding their pressure dependence.
In this work, at 1550 nm, we investigate the hydrostaticpressure dependence of the Bragg wavelength of a PFGI-POFFBG for the first time to the best of our knowledge.
II. FBG I NSCRIPTION AND M EASUREMENT S ETUP
A 2-mm-long FBG was inscribed in a 1.3-m-long PFGIPOF. The PFGI-POF (GigaPOF-50SR, Chromis Fiberoptics)
has a three-layered structure: core (diameter: 50 μm; refractive index: ∼1.35), cladding (diameter: 70 μm, refractive
index: ∼1.34), and overcladding (diameter: 490 μm). The
core and cladding consist of doped and undoped amorphous
fluoropolymer (CYTOP®), respectively, the water absorptionof which is negligibly small [27], and the overcladding iscomposed of polycarbonate. The optical propagation loss isrelatively low (∼0.25 dB/m) even at 1550 nm. The numericalaperture was ∼0.19. The FBG was inscribed without removalof the overcladding using a femtosecond laser system (High Q
femtoREGEN, High Q Laser) operating at 517 nm with apulse duration of 220 fs. The laser was operated at a repetition
rate of 1 kHz, for a pulse energy of ∼100 nJ [20], [28]. The
PFGI-POF was mounted on an air bearing translation system(Aerotech); this allowed high-resolution and high-accuracy
motion on two axes. A long working distance objective (x50)
was mounted on a third axis, and the laser beam was focused
into the fiber. Accurate synchronization of the laser pulse
repetition rate and the stage motion allowed for plane-byplane grating inscription, enabling the grating to have a desired
length and an index-modulation value [20], [22], [23], [28].
Figure 1(a) shows an experimental setup for measuring the
Bragg wavelength of the FBG. All the optical paths except
the PFGI-POF were silica SMFs. The output from an ASE
source was injected into the PFGI-POF, and the reflected lightfrom the FBG was guided via an optical circulator to an
optical spectrum analyser (AQ6370, Yokogawa Electric Corp.;

Fig. 3. FBG-reflected spectra around the sharp dip measured at 0.1 MPa(blue) and 0.5 MPa (red).

stability: ±0.02 nm in the range from 1520 to 1580 nm;
reproducibility: ±0.005 nm in 1 min). We employed a reflectometric (not transmissive) configuration to minimize the influence of modal interference [29]. The PFGI-POF was placedin a hydraulic system (MN151-001, Think-Lands; Fig. 1(b)),
in which the hydrostatic pressure P can be controlled from
0.1 MPa (atmospheric pressure) to 0.5 MPa. One end of the
PFGI-POF was connected to a silica SMF using a so-called
butt-coupling technique (SMF-to-POF coupling loss: ∼0.2 dB;
theoretical returning loss: −28 dB) [30], and the other end was
kept open. Note that the Fresnel reflection at the open end is
automatically suppressed because the refractive index of wateris ∼1.33, close to that of the fiber core (∼1.35).
III. E XPERIMENTAL R ESULTS
We first measured the optical spectra of the ASE output
and the FBG-reflected light, as shown in Figs. 2(a) and (b).
The spectrum of the light Fresnel-reflected mainly at the
SMF-to-POF boundary was overlapped with the FBG-reflected
spectrum, which was still clearly observed at 1559 nm. The
magnified view of the FBG-reflected spectrum around its peaks
is shown in Fig. 2(c). Multiple peaks and dips, caused by
the multimode nature of the POF [31], were observed in the
spectrum. The wavelength of any peak or dip (including thehighest peak) could be selected as a Bragg wavelength, buthere, the clear and sharpest dip at 1558.97 nm was used toenhance the measurement accuracy.

ISHIKAWA et al.: PRESSURE DEPENDENCE OF FBG INSCRIBED IN PERFLUORINATED POLYMER FIBER

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Fig. 4. (a) Temporal dependence of the FBG-reflected spectrum at 0.5 MPa. (b) Temporal dependence of the Bragg wavelength at 0.5 MPa. (c) FBG-reflected
spectrum measured after 0.5-MPa pressure was applied for 240 min. The red-circled parts are the dip (right) and the peak (left) used in measurements.

Fig. 5. (a, b) Pressure dependences of FBG-induced spectra around the sharp peak when the pressure was decreased from 0.5 MPa to 0.1 MPa and when
the pressure was then increased from 0.1 MPa to 0.5 MPa. (c) Measured Bragg wavelength dependence on pressure; the red points indicate the data when
the pressure was decreased, and the blue points are the data when the pressure was then increased. The two lines are their linear fits, which are overlapped
and appear to be one line in this figure.

Subsequently, we measured the FBG-reflected spectra
around the dip before and after the pressure P was abruptly(within 20 s) increased from 0.1 MPa (atmospheric) to
0.5 MPa, as shown in Fig. 3. Although the linewidth ofthe dip was slightly reduced, the Bragg wavelength shift
was negligibly small. After that, however, as the pressure P
was maintained at 0.5 MPa, the Bragg wavelength shifted
as time proceeded, as shown in Fig. 4(a). Figure 4(b)shows the temporal dependence of the Bragg wavelength at
0.5 MPa. The Bragg wavelength increased with time and,approximately 150 min later became almost constant. The
total Bragg wavelength shift was approximately 0.5 nm. The
positive dependence of the Bragg wavelength on pressure was
the same as those of other types of POFs [16], [17], but the
wavelength shift accompanying a time constant of over several
tens of minutes was first observed in this measurement. Sucha long time constant appears to be caused by the structure of
the PFGI-POF, which is composed of two different polymer
materials (CYTOP (core and cladding; pressure strength:30 MPa) is known to be mechanically much weaker than polycarbonate (overcladding; pressure strength: 86 MPa)). If weconsider that the Bragg wavelength dependence on pressure
without considering the time constant (i.e., when the Bragg
wavelength is measured at each pressure after waiting for a

sufficiently long time for the signal to be constant) is linear,
the dependence coefficient is estimated to be ∼1.3 nm/MPa(corresponding to ∼860 × 10−6/MPa). This value is ∼5.2
and ∼13 times larger than those of the FBGs in an SMPMMA-POF [16] and MM-mPOF [17], respectively. This
result indicates that, by removing the overcladding of the
PFGI-POF in advance, higher-sensitivity pressure sensing
with a short response time could be achieved. Note that the
linewidth of the dip increased with time, and compared with
Fig. 2(c), the FBG-reflected spectrum was so much changed(Fig. 4(c)) that the dip was no longer sharp, therefore weused the highest peak for the following measurements.
Finally, the pressure dependence of the Bragg wavelength(measured using the highest peak, as mentioned above) was
measured in a short period (<1 min). Figures 5(a) and (b)show the pressure dependences of the FBG-reflected spectra when the pressure P was decreased from 0.5 MPa to
0.1 MPa and when P was then increased from 0.1 MPa to
0.5 MPa, respectively. In both cases, as the pressure became
higher, the Bragg wavelength grew shorter. The spectral powerchange seems to have been caused by the multimode natureof the POF. The Bragg wavelength was then plotted as afunction of pressure P, as shown in Fig. 5(c). Irrespectiveof the direction of the pressure change, the Bragg wavelength2170

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 29, NO. 24, DECEMBER 15, 2017exhibited a negative dependence on pressure with a coefficientof −0.13 nm/MPa (standard deviation: <2.72 pm; corresponding to a fractional sensitivity of −84 × 10−6 /MPa). Thisabsolute value is ∼42, ∼0.52, and ∼1.3 times larger than thoseof the FBGs in a silica SMF [15], SM-PMMA-POF [16], and
MM-mPOF [17], respectively. The negative dependence,
which is opposite to those of other types of POFs [16], [17], iscaused by the fact that the contribution of the elongated grating
pitch is greater than that of the reduced refractive index inthis short-term measurement. The drawing-induced anisotropy[17], [32] and three-layered structure of the PFGI-POF mayneed to be considered for further analysis. The imperfectly
linear pressure dependence of the Bragg wavelength probably
originates from the multimode nature of the POF, but as longas the dependence is monotonic, it can be used to performpressure sensing by exploiting the one-to-one correspondence
between the Bragg wavelength and pressure.
IV. C ONCLUSION
The hydrostatic pressure dependence of the Bragg wavelength of an FBG inscribed in a PFGI-POF was investigatedat 1550 nm. The Bragg wavelength shift was negligibly small
shortly after the pressure was abruptly increased to 0.5 MPa.
However, when the FBG was maintained at 0.5 MPa, theBragg wavelength increased with time and became almost
constant ∼150 min later. The long time constant seems
to have been caused by the overcladding. A simply calculated pressure-dependence coefficient was 1.3 nm/MPa, which
is more than 5 times the values of other types of POFFBGs. The pressure-dependence coefficient was measured
at −0.13 nm/MPa. Unlike other POF-FBGs, the sign ofthe pressure-dependence coefficient was negative while theabsolute value was not significantly different. Our results may
indicate the feasibility of high-sensitivity pressure sensing
with a short response time by removing the overcladding of
the PFGI-POF-FBG.
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